1. Field of the Invention
This invention relates to new products and processes for the non-impact recording of visible signals and images and applies in particular to the printing of alphanumerical and graphical signals produced by computers, measurement devices, etc., to recording and hard-copy print-out of images and data such as displayed on video terminals, to facsimile and telecopy transmission of documents and images.
2. Brief Description of the Prior Art
A large number of non-impact recording processes are known: photographical, electrostatic, thermal, laser, sparking, electrolytic, etc., which use different physical and chemical processes to produce visible marks on a recording material.
Among the known processes are those which use the flow of electrical current through the recording medium to produce marks. These processes are of special interest since they fit most easily to electronic means and use the simplest writing or printing heads. Using one or more conductive styli they offer significant advantages from the point of view of simplicity, cost, size and reliability of the recording device.
One can distinguish two prinicipal kinds of processes using the flow of electric current through the recording medium.
The first kind are called the indirect or development processes, in which the current flow is used to create a latent image or low density image which must eventually be revealed or developed through a thermal chemical or physical process. Such processes using thermal development are described in the French Pat. No. 2,280,517, in the British Pat. No. 1,275,929, and in the U.S. Pat. Nos. 4,155,760, and 4,155,761. In the first named patent, a writing electrode to which a voltage of several kilovolts is applied creates a latent image in a sensitive layer coated on a conductive layer and containing a reducible metal compound, such as silver, nickel or copper salt of an organic acid; and a reducing agent. A visible image is then developed by uniformly heating at a temperature of between 80.degree. C. and 250.degree. C. This allows the reduction of the metal compound by the reduction agent, around the nuclei forming the latent image. In the second named patent, the metal compound generating the latent image may also be the metal compound generating the visible image (a nickel, cobalt, zinc, chromium, tin or copper compound), or be different from it (an aluminum, manganese, platinum, palladium or gold compound). In the third and fourth patents named above, a reducible coordination complex of tellurium (II) is used to generate the latent image and then the visible image. With such processes, the production of a visible image requires the use of a heating device in addition to the writing device. This increases complexity, cost, size and energy consumption of the apparatus used. In addition further processing is needed to destroy or deactivate the residual reducing agent. Otherwise a progressive discoloration of the background is experienced. Also, when the final image is not a silver one it may also be necessary to protect it from the oxidizing action of the air, for instance by coating it with a synthetic resin layer.
Another indirect process (using chemical development) is described in the U.S. Pat. No. 3,425,916. A latent image is generated, as in the previously described processes, from various metal compounds. However, the development of the visible image results from the chemical reduction, in the wet state, of a silver salt. This visible image must be stabilized by washing and fixing, as in the conventional photographic processes. In addition to the complexity of the wet processing device, it should be noted that a silver-based recording medium is very costly.
Another category of indirect processes described in the patent literature is characterized by the imagewise deposition, with writing electrodes, of electrical charges on a recording sheet coated with an insulating layer. The latent image is then developed into a visible image by electrostatic attraction of particles of a colored powder (the toner) which are eventually secured to the sheet by heating, which melts a resin contained in the toner particles. These processes, derived from xerography, contemplate a complex series of processing steps to get the final image and need a bulky device with many costly mechanical parts. Also, adding to the cost of such processes are the electrode high voltage controls and the scanning electronics required.
The second kind of process are the so-called direct processes, in which the electric current flow produces a visible image, without the need of further development. Such processes are recording devices much simpler than the indirect processes. Such devices are cheaper and can be miniaturized. A first category of direct processes are the sparking processes (also called electrothermal, or dry electrosensitive), in which the recording medium possesses a black conducting layer coated with white-greyish pigments which may be volatilized by a spark generated between a writing stylus and the black layer. This process produces nuisances for the environment, i.e., noise, fumes, odor and RF interferences. In addition, the marks have fuzzy limits is greyish and opaque, does not look like plain paper. Another paper used in direct sparking processes possesses a non-conducting black layer coated with a thin aluminum conducting layer, which is volatilized by the spark. The resolution is better with its paper but contrast is highly penalized by the metallic glossy appearance, which makes reading rather uncomfortable.
Another category of direct process is found in the electrochemical processes (also called electrolytic, electrochromic, etc.) which use electric current flow to produce an electrochemical and/or chemical reaction, generating a colored product. These processes generally use a low voltage and do not generate noise, fumes or RF interference.
An important family of electrochemical processes use a recording medium a paper impregnated with an electrolytic solution and kept wet until use or rewetted just prior to use. In use it is clamped between a writing electrode and a counter-electrode. A large number of electrochemical reactions can be used with this paper in such processes. Representative of such reactions are:
1. Cathodic reduction or anodic oxidation of a dissolved species, for instance oxidation of a colored redox indicator such as leucorosaniline, or oxidation of iodide ion into iodine, forming with starch the well-known blue complex;
2. Anodic oxidation of the writing electrode and reaction of the generated ions with a color intermediate present in the electrolyte, for instance anodic oxidation of a silver electrode and reduction into metal of the silver ions by a reducing agent such as pyrogallol or ascorbic acid, or anodic oxidation of an iron electrode and generation of a color-lake by reaction of the iron ions with pyrocatechol;
3. pH change at the vicinity of the writing electrode, triggering a color-forming reaction between colorless products, for instance a coupling reaction between a diazo ccompound and phenol.
It will be appreciated that this family of electrochemical processes has only a very specific range of applications. In addition to the need to keep the paper wet in a sealed bag or to rewet it, and to dry it after recording, the image resolution is very poor. The colored reaction develops between or under the effect of dissolved species which are free to diffuse, and therefore the mark tends to "bleed" out. In addition to this, the mark fades with time or the background discolors. Lastly, many of the chemicals used are irritating or even toxic and/or have an unpleasant odor.
Attempts have been made to remedy certain of the above-described drawbacks, especially the poor resolution and the need of wetting the recording medium in another family of electrochemical processes which are often more specifically called "electrochromic". Here, the electrosensitive layer contains an insoluble compound, often a metal compound, preferentially colorless, white or slightly colored. These compounds, called electrochromic compound or material are converted by anodic oxidation or, more frequently, by cathodic reduction, into a different valency, often the free metal state, where it is both colored and insoluble. The mark formed cannot bleed out by diffusion. The valency change occurs within each solid particle or crystallite brought to the suitable potential. The recording medium is generally formed by a layer of the electrochromic material particles dispersed in a binder matrix and coated on a conducting layer (aluminum, tin dioxide, carbon, etc.), itself coated on a base paper. The base paper compensates for the very low ionic conductivity of the electrosensitive layer (itself due to the low ionic conductivity of most of the electrochromic compounds) and is used as counter-electrode. The writing electrode, generally a stylus, is moved on the free surface of the electrosensitive layer. Several variants of this process are described for instance in the U.S. Pat. Nos. 3,138,547 and 4,199,413 and in the French Pat. Nos. 2,341,884, 2,398,331 and 2,435,100 which describe many electrochromic solids. Many are reducible into free metals, i.e.; ZnO, PbO, PbCl.sub.2, basic Pb CO.sub.3, SnO.sub.2, Sn (OH).sub.2, Sn(II)oxychloride, Ni.sub.2 O.sub.3, NiO, CdO, Bi.sub.2 O.sub.3, Ceo, V.sub.2 O.sub.5, Sb.sub.2 S.sub.5, H.sub.2 MoO.sub.4, MoO.sub.3, W o.sub.3, Zr Ti O.sub.4, AgO, etc. However, some of these compounds barely react. Many are colored and/or produce a mark which is not black. Forinstance, Mo O.sub.3 is yellow and produces a blue mark; Sn (OH).sub.2 and Sn(II)oxychloride are yellow and produce a black-brown mark which loses contrast if the medium is not treated with EDTA or ammonia after recording. The plain paper appearance of the recording medium, already affected by the presence of the opaque conducting layer, may also be jeopardized by the background color. It should also be noted that except for the costly silver compounds, most of the electrochromic compounds generate a mark which reoxidises in the presence of air. Associating particles of several different electrochromic compounds can only produce a juxtaposition of metal particles and not an alloy which would be more resistant to corrosion than its components.
A special kind of electrochromic process which also needs a base material sheet coated with an electronically conducting layer, uses as an electrochromic material a solution of a metal salt which is reduced and plated as free metal on the conducting layer. The conducting layer acts as a fixed cathode. In the U.S. Pat. No. 3,010,883, the electrolyte, which contains CuSO.sub.4, AgNO.sub.3 or NiCl.sub.2, must be applied as a free solution or as a gelatinized film just prior to recording and removed just after. It can also be solidified with polyethylene glycol and coated on the cathode, but must be rewetted or melted just prior to recording. In addition to the problems created by the existence of the conducting layer and by the special glossy appearance of the plated metal marks, the recording device is very complex.
The present invention relates to a new electrochemical process and new materials for the electrical recording of visible signals, avoiding the drawbacks of the processes and materials of the prior art described hereinabove, especially of the electrochemical, electrochromic and related processes.
More specifically, one object of the invention is to provide process for recording signals and images which are directly visible, stable and permanent with high resolution and high contrast image. This is done by imagewise flowing of an electric current between at least one writing elecrtrode and a recording medium electrically connected to at least one second electrode or counter-electrode, without the need of any additional development or fixing processing, and without the generation of sparks, fumes or odors.
Another object of the invention is to provide an electrochromic material having a high ionic conductivity, a continuous non-grannular texture and a mechanically solid consistency, which is able to be coated on a substrate or a base material as a continuous layer or film, and able, by contacting its free surface with a fixed or mobile electrode brought at a negative electric potential, to have metal ions present in the material cathodically reduced within the material at the contact point into directly visible, stable metal marks of black color, high optical density and sharp edges, without the generation of sparks, fumes or odors.
Another object of the invention is to provide a recording medium capable of permanently demonstrating, in a wide range of atmospheric humidities, the electrosensitivity necessary to record directly visible images, without the need of an activation or sensitization treatment prior to recording.
Another object of the invention is to provide a permanently electrosensitive medium able to demonstrate the surface appearance feel, color, opacity or transparency specific to the employed substrate or base material, in order to preserve the natural appearance of this substrate or a useful physical feature of it. More specifically, in case of the use as substrate of plain cellulose paper, able to preserve the surface appearance, the partial translucence, the white color and the natural feel of such paper. More specifically again, in the case of use of a transparent substrate, able to preserve this transparency in order to allow rear projection and rear observation of the recorded images.
Another object of the invention is to provide an inexpensive recording medium containing no toxic or environmentally harmful component. This medium may be manufactured by coating or impregnation, followed by drying, of a substrate or base material in the form of a sheet with a liquid or fluid mixture which is inexpensive and easy to prepare.
Another object of the present invention is to provide a recording medium for generating images whose background and marks are unaffected by light, heat and humidity.
Another object of the invention is to provide a recording process using one or more writing electrodes, more particularly shaped as styli.
These objects and others will be better understood with the following description of the invention.